The present invention relates to a gate valve used for opening and closing of a vacuum chamber and particularly to a gate valve in which a gate comes in contact with and separates from a rectangular valve seat.
As a gate valve for opening and closing a flow path between chambers, there are conventionally known valves as disclosed in Japanese Patent Application Laid-open No. 9-303577, Japanese Patent Application Laid-open No. 10-227365, and the like, for example, in which a gate is mounted to a tip end of a shaft driven by a cylinder device and the gate is pressed against a valve seat mounted around a valve opening by tilting the shaft after moving the gate to a position facing the valve opening by the shaft.
In the gate valve, a sealant 33 at a periphery of the rectangular gate 32 is normally brought into contact with and separated from the valve seat 31 provided around a rectangular valve opening 30 as shown in FIGS. 4 and 5 because of a flow path section between the gate valve and the vacuum chamber and the like. Therefore, if a central portion of a longer side of the gate 32 is fixed to the shaft 34 and the gate 32 is brought into contact with and separated from the valve seat 31, flection xcex4 is generated at opposite ends of the gate 32 as shown with a two-dot dashed line in FIG. 4 when the gate valve is upsized and a length L between seal portions on a side of the longer side of the gate 32 is large to some extent, though there is no problem when the gate is relatively small.
In other words, in the gate valve, a reaction force applied to the gate 32 from the valve seat 31 when the sealant 33 of the gate 32 receives a force F of the shaft 34 and is pressed against the valve seat 31 is obtained as the product of a load per unit length on a seal line and a length of the seal line. In this case, the product of the seal length M and the load per unit length acts as a concentrated load on each of opposite end portions of the longer side of the gate 32 while the load acts as a uniform load on a portion of the length L in a longitudinal direction of the gate 32. Therefore, the load acting on the gate 32 is distributed such that the concentrated load acting on each of the opposite end portions of the longer side where the maximum bending moment is applied becomes extremely large and the opposite end portions are the most likely to be distorted due to stress and bending.
If the flection xcex4 is generated in the opposite end portions of the gate 32 as described above, a sealing property of the sealant 33 is impaired. Therefore, measures are taken in such a manner that a thickness of the gate 32 is increased to increase mechanical strength to thereby increase second moment of area or in such a manner that the shaft 34 is divided into a plurality of connecting portions in a fork shape or a plurality of shafts are provided, for example, to thereby distribute fulcrum portions to substantially reduce the length L and to reduce the flection at the opposite ends.
If the thickness of the gate 32 is increased, a thickness of the gate valve itself increases, an overall weight increases, and vibration increases in opening and closing of the gate. If the gate 32 is retained at a plurality of positions by means of dividing the shaft 34 into the plurality of connecting portions and the like, a structure for this purpose becomes complicated and cost increases and a vacuum property may be impaired in some cases while dimensions of the gate valve can be reduced relatively.
The present invention aims to solve the above-described problems and it is a technical object of the invention to provide a gate valve in which generation of flection xcex4 at opposite end portions due to bending moment acting on the gate can be easily suppressed in the above gate valve.
It is another technical object of the invention to provide a gate valve in which a mounting structure of a gate and a shaft to each other is improved and second moment of area of the gate is increased to thereby reduce flection at opposite ends of the gate.
It is another technical object of the invention to provide a gate valve in which weight of the gate is reduced by enhancement of mechanical strength of the gate and an amount of deformation of the gate when a seal is provided is reduced to evenly apply a sealing force with a small valve closing force to thereby carry out sealing with a small amount of leakage.
It is yet another technical object of the invention to provide a gate valve in which, because it is unnecessary to strongly press the gate against the valve seat, it is possible to use a small-sized cylinder device for pressing the gate against the valve seat with a small force and therefore it is unnecessary to increase overall member strength.
To achieve the above objects, in a gate valve of the invention, a central portion of a longer side of a rectangular gate being mounted to a tip end of a shaft operated by driving of a cylinder device and the gate being pressed against a valve seat provided around a rectangular valve opening by tilting the shaft with tilting means after the gate is moved to a position facing the valve opening by driving the shaft with the cylinder device, the gate is formed to have an uneven wall thickness including a large thickness in a vicinity of a fulcrum portion connected to the shaft and a small thickness on each of opposite end portion sides.
In the above gate valve, it is effective that a connecting hole for connecting the shaft for driving the gate is formed in a fulcrum portion of the gate, the connecting hole is formed such that a thickness of the gate on a side of a contact face with the valve seat is large and that a thickness of the gate on a side of an opposite face is small, and the tip end of the shaft is inserted into the connecting hole to connect the gate to the shaft.
In the gate valve having the above structure, because the thickness of the fulcrum portion at the center of the gate which receives the maximum bending stress due to the force from the shaft is increased to give the gate an uneven wall thickness for suppressing flection at the opposite end portions due to the bending moment, the mechanical strength of the gate itself is effectively enhanced and the gate in which flection of the opposite end portions is small can be formed as a lightweight gate. Because the connecting hole is formed in the gate in connecting of the gate and the shaft and because the thickness receiving tensile stress on the side of the contact face with the valve seat is increased and the thickness on the opposite side receiving compression is reduced to increase the second moment of area, it is possible to further reduce flection of the opposite end portions of the gate.
Moreover, because an amount of deformation of the gate when a seal is provided is reduced by enhancement of the mechanical strength of the gate, it is possible to evenly apply a sealing force with a small valve closing force to carry out sealing with a small amount of leakage. Furthermore, because it is unnecessary to strongly press the gate against the valve seat, it is possible to use the small-sized cylinder device and it is unnecessary to increase overall member strength.